Shifts and Shielding Effects

Chemical Shifts and Shielding Effects.—Phosphorus-31. The positive shifts ( 5p) which are reported in this chapter are upfield from 85 % phosphoric acid. 

Chemical Shifts and Shielding Effects.—Phosphorus-31. The sign convention used for expressing shifts in this Report is not the same as was used in earlier volumes. Positive chemical shifts are now downfield from 85 % phosphoric acid, and are given without the appellation p.p.m. Since both conventions are in use, it remains necessary to state the sign convention used in each paper published. 

Lebedev and A. I. Rezvukhin, Izvest. sibirsk. Otdel. Akad. Nauk S.S.S.R., Ser. khim. Nauk, 1975, 149. 

Compounds. Theoretical estimates of 5p by CNDO/2 calculations required the inclusion of phosphorus /-orbitals and an adjustable parameter which depends on the type of compound, e.g. phosphine or phosphite. The chemical shifts of a series of cyano-compounds (5 X, Y=hal or CN) agree with those predicted by Letcher and van Wazer s quantum-mechanical interpretation. The sensitivity of 5p to stereochemical changes often leads to quite large differences of chemical shift between various conformers or isomers, e.g. the axial conformer (6) has 5p upheld 

Chemical Shifts and Shielding ElSects.—Phosphorus-31. In this section, positive P chemical shifts (dp) are upheld from 85% phosphoric acid. A linear relationship between dp and vapour pressure for elemental phosphorus in the gas phase has been reported. Solvent effects have also been examined. dp compounds. The n.m.r. parameters of various protic and deuteriated 

There has been a marked increase in the use of P n.m.r. spectroscopy. The improvements in instrumentation, such as for Fourier transform, have increased the scope of this aspect of n.m.r. considerably. Thus there has been a marked advance in the study of phosphorus-containing natural products. It is now possible to show that in solution (1) predominates ca. 90%) over its cw-isomer (jS-furanose), with negligible amounts of the open-chain forms. Also, well-structured P n.m.r. spectra 

has been used to study the structure of solids. - Although the signals are necessarily broad, quite accurate estimates of Sp have been obtained, e.g. solid tetrabutylphosphonium iodide has Sp —35.1 p.p.m., which may be compared with — 32 p.p.m. for a solution. 

There are many examples of shielding of the phosphorus nucleus caused by the introduction of electronegative groups on a substituent which may 7r-bond with the phosphorus atom. A recent example concerned the phosphonates (2). This has usually been attributed to an increase in 

However, there are observations which do not appear to be explained by this theory. For instance, in comparison with a phosphonium salt, the phosphonium ylide possesses considerable d -p bonding and has a less electronegative carbon atom bonded to phosphorus. It is predicted that both of these effects should deshield the phosphorus nucleus, which is contrary to the experimental observations. Further, when the carbanion of the ylide is delocalized (which may be achieved with a minimal change in inductive effect), Sp is shifted downfield, and when alkyl groups are introduced on the a-carbon atom of ylides, Sp is shifted upheld. Note that the alkyl groups would be expected to increase the +/ effect and the + M effect by way of increased /7-character of the lone pair of electrons. Similar conclusions may be drawn from a comparison of ylides with phosphine oxides. 

Emsley, J. Feeney, and L. H. Sutcliffe, High Resolution Nuclear Magnetic 

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Applications including Chemical Shifts and Shielding Effects... 

Chemical Shifts and Shielding Effects.—Chemical shifts are usually given without the appellation p.p.m. The scope of n.m.r. pulsed methods has been reviewed. ... 

Applications including chemical shifts and shielding effects... 

Figure 9.11—Relationship between chemical shifts, magnetic field, and shielding effects (proton NMR).

The AB and AX systems of all 13C —13C bonds appear in one spectrum when the INADEQUATE pulse sequence (Fig. 2.48) is applied. Complete interpretation usually becomes difficult in practice due to signal overlapping, isotope shifts and AB effects (Section 2.9.4). A separation of the individual 13C— 13C two-spin systems by means of a second dimension would be desirable. It is the frequency of the double quantum transfer (d e) in Fig. 2.48 which introduces a second dimension to the INADEQUATE experiment. This double quantum frequency vDQ characterizes each 13CA — I3CX bond, as it depends on the sum of the individual carbon shieldings vA and vx in addition to the frequency v0 of the transmitter pulse located in the center of the spectrum if quadrature detection is applied [69c, 71] ... 

A series of silanol and silylamine chemical shifts were obtained in various solvents. (83) The silanols are found to be highly dependent (>5ppm shifts) upon solvent basicity with the more basic solvents causing low frequency shifts. This shielding effect is found to give an excellent linear correlation with Gutmann s donor number (DM) (130) which is a measure of the electron pair donor ability of the solvent. Figure 23 shows the correlation for five of the compounds examined. It... 

Chemical Shifts. The shielding effect of spherically symmetrical s electrons is discussed in Section 18.3.2.1. This diamagnetic upfield shift affects all nuclei since every molecule has s electrons. For electrons in /t-orbitals there is no spherical symmetry and the phenomenon of diamagnetic anisotropy is used to explain some otherwise... 

It is only in the past decade that extensive studies, fundamentally theoretical calculations of structure and energy, and experimental dynamic NMR and shielding effect calculations have been performed to know the contribution of each conformer and that of the average to the NMR shift that is observed. 

The traditional interpretation of the isomer shift in Fe Mossbauer spectra is based on the following assumptions (a) the influence of the 3d electron configuration on the IS occurs via the shielding effect of the 3d electrons on the 3s and 4s electrons, (b) the variations in the 3s shell are dominant, and (c) the influence of the 4s shell occurs via the 4s population. 

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